Eye-related measurements should be understood generally to mean measurements which determine properties of a person's eyes in isolation or measure some other object, in particular a spectacle lens, vis a vis a person's eyes. One example of an apparatus for measuring properties of the eyes themselves is a refractometer for objective refraction measurement, which determines the refraction of eyes at a position to be examined and outputs it usually in the form of sphere, cylinder and axis, as defined in DIN ISO 13666: 2012. Another example is a perimeter used to determine a person's field of view (cf. “Perimetry” in the Wikipedia Article “Visual field test,” last accessed Jul. 31, 2018). A further example is an apparatus for generating sectional images of the anterior chamber or the retina of an eye, for example by means of optical coherence tomography (OCT); see Wikipedia article “Optical Coherence Tomography,” last accessed Jul. 31, 2018. Further examples include apparatuses for biometry of the eye, which allow the selection of a patient-specific intraocular lens before a cataract operation, i.e., the selection of a lens to be inserted into the eye. An apparatus of this type is sold by the Carl Zeiss® Group under the designation IOL Master®. By means of such eye-related measurements that determine properties of the eye itself, measurement data (e.g., the refraction mentioned above) describing a state of the eye are thus obtained. On the basis of the measurement data, a physician or optician can then make a diagnosis by checking whether the measurement data indicate a syndrome. In the case of refraction, the diagnosis can then amount e.g., to short-sightedness or long-sightedness. This establishment of the diagnosis is thus a further procedure after the provision of the measurement data by the eye-related measurement.
One example of an apparatus that measures a spectacle lens relative to an eye for the purpose of positioning the spectacle lens is a so-called centring apparatus. Centring apparatuses are described in WO 2005/069063 A or in European Patent Applications Nos. EP 3 355 102 A1 and EP 3 355 100 A1. Centring apparatuses determine so-called centring parameters that are used by an optician for correct seating by grinding and adaptation of the spectacle lens, as explained in greater detail in these citations.
An overview of various apparatuses for refraction and centring measurement is also given by the web page www.zeiss.com/vision-care/en_us/products.html, last accessed Jul. 31, 2018. The related art discloses a wealth of further examples for the apparatuses mentioned above. Therefore, the latter will not be explained in any greater detail here.
In the case of such apparatuses, for measurement purposes it is often necessary for the person who is to be examined to adopt a specific viewing direction. By way of example, in centring measurements it is often assumed that the person has directed the gaze horizontally substantially into infinity.
To ensure that the person adopts a viewing direction required for the respective measurement, a fixation target is provided. In the context of the present application, a fixation target is understood to mean a real or virtual object or image at which the person is supposed to direct his/her gaze during the respective measurement. In this case, the object or image is virtual if it is not an actual image or object, but rather for example a spatially projected image or a mirror image.
Many apparatuses, e.g., the apparatus for 3D video measurement as shown at www.rodenstock.us/us/en/lenses/rodenstock-technologies/3d-video-measurement.html, last accessed Jul. 31, 2018, operate such that the person to be examined fixates an object, the mirror image of the person himself/herself, or a light-emitting diode fitted to the apparatus, from a relatively short distance, e.g., 1 m. A video camera then records images of the person while the latter views the fixation target thus formed. What is disadvantageous about that is that if the person takes up his/her gaze at the fixation target, the eyes adopt a so-called convergence position, i.e., are not directed into infinity. Instead, the viewing direction of both eyes extends towards the point of the fixation target, which is situated relatively close to the person (for example at the abovementioned distance of 1 m). The purely geometric convergence resulting from the position of the eyes and of the fixation target can be compensated for computationally in this case. As explained in U.S. Pat. No. 7,384,144 B2, however, this geometric convergence does not necessarily correspond to the actual convergence of the pair of eyes, that is to say that the actual position of the eyes when viewing the fixation target can deviate from the position determined from purely geometric considerations. This can in turn lead to inaccuracies during the measurement.
Therefore, U.S. Pat. No. 7,384,144 B2 proposes an apparatus for generating a fixation target, in which a speckle pattern projected into infinity is generated by means of a laser light source through a diffractive element. For this purpose, in the case of the apparatus in U.S. Pat. No. 7,384,144 B2, the laser light is projected onto a screen via a diffraction element to form a speckle pattern on the screen, and the pattern is then projected into infinity by an optical system. By way of example, the optical system can realize a magnifying glass imaging by means of a lens or lens group. In this case, a diffractive element is an element that works on the basis of light diffraction, in contrast to refractive elements such as lenses, which work on the basis of light refraction. The speckle pattern can be superimposed with an additional pattern, for example a cruciform pattern, as likewise explained in U.S. Pat. No. 7,384,144 B2.
By virtue of the speckle pattern being projected into infinity, the person basically adopts a viewing direction for which the gaze is directed into infinity. Therefore, there is no need to carry out any compensation on account of a convergence of the eyes to a comparatively closely situated point.
Nevertheless, the fixation target in U.S. Pat. No. 7,384,144 B2 still has disadvantages. Firstly, there are persons who, on account of the subjective proximity of the apparatus, exhibit a type of residual convergence, that is to say that the eyes do not exactly adapt to the viewing direction of infinity.
In addition, in the case of the apparatus in accordance with the related art, the image is fixated by both eyes (binocular vision), wherein the typical distance between the eyes is approximately 64 mm. Therefore, the optical system that generates the fixation target on the basis of the laser light is used outside its optical axis. Here, in the case of a rotationally symmetrical optical system, the optical axis is the axis of symmetry of the system and passes in particular through centers of curvature of curved surfaces. Optical systems have imaging aberrations such as spherical aberration or distortion, which prove to be more severe the further away from the optical axis. The imaging aberrations can be reduced by various measures, for example by the use of aspherical lenses, but this increases the costs.
Moreover, the apparatus in accordance with U.S. Pat. No. 7,384,144 B2 is relatively large. Since a traditional optical system comprising refractive elements, in particular lenses, is used within this apparatus, a specific distance is maintained between the refractive elements used and a screen onto which the laser light is projected as a speckle pattern. The distance corresponds approximately to the focal length of the lens. The shorter the focal length of the system, the greater the extent to which imaging aberrations become visible. On the other hand, larger focal lengths increase the structural space required.
Finally, the apparatus in accordance with U.S. Pat. No. 7,384,144 B2 is comparatively expensive. In the case of the apparatus in U.S. Pat. No. 7,384,144 B2 for generating the fixation target use is made of at least one laser, a diffractive element for generating a pattern to be projected (in particular a speckle pattern), a screen, and an imaging optical unit, typically consisting of one or more lenses. Each of these components contribute to the financial outlay.
US 2013/0201446 A1 discloses a hologram that encodes one or more holographic images. The holographic images can be used as eye charts or for eye training. In this case, the holographic images can be generated substantially at infinity or at a finite distance. Switching between the holographic images in the case of a plurality of holographic images is not explained in this case. Moreover, the use of such holographic images as a fixation target is mentioned.